Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor

Abstract

Despite decades of debate, it remains unclear whether human bipedalism evolved from a terrestrial knuckle-walking ancestor or from a more generalized, arboreal ape ancestor. Proponents of the knuckle-walking hypothesis focused on the wrist and hand to find morphological evidence of this behavior in the human fossil record. These studies, however, have not examined variation or development of purported knuckle-walking features in apes or other primates, data that are critical to resolution of this long-standing debate. Here we present novel data on the frequency and development of putative knuckle-walking features of the wrist in apes and monkeys. We use these data to test the hypothesis that all knuckle-walking apes share similar anatomical features and that these features can be used to reliably infer locomotor behavior in our extinct ancestors. Contrary to previous expectations, features long-assumed to indicate knuckle-walking behavior are not found in all African apes, show different developmental patterns across species, and are found in nonknuckle-walking primates as well. However, variation among African ape wrist morphology can be clearly explained if we accept the likely independent evolution of 2 fundamentally different biomechanical modes of knuckle-walking: an extended wrist posture in an arboreal environment (Pan) versus a neutral, columnar hand posture in a terrestrial environment (Gorilla). The presence of purported knuckle-walking features in the hominin wrist can thus be viewed as evidence of arboreality, not terrestriality, and provide evidence that human bipedalism evolved from a more arboreal ancestor occupying the ecological niche common to all living apes.

Fig. 1.

Ontogenetic morphology of putative knuckle-walking features in African ape scaphoids. Proximolateral view of left scaphoids. Juvenile specimens are labeled by developmental stage (Table S1) and all other specimens are adult. Both the dorsal concavity (A) and beak (B) of the scaphoid's radial facet appear earlier and are more common and accentuated throughout ontogeny in P. troglodytes and P. paniscus. These features are rarely found in Gorilla and instead the corresponding area is round and convex (all specimens to 1 cm scale).

Fig. 2.

Ontogenetic morphology of putative knuckle-walking features in African ape capitates and hamates. Dorsal view of right capitate (Above) and hamate (bBelow). Juvenile stages are labeled and all other specimens are adult. Black dashed line shows less capitate waisting at all stages of ontogeny in Gorilla compared to Pan (see Fig. S2). The capitate's distal concavity (A) and dorsal ridge (B) appear earlier and are more common and accentuated in Pan but are frequently absent in Gorilla. In Gorilla the hamate's distal concavity (C) is shallower and the dorsal ridge (D) appears earlier in ontogeny but is again less frequent and accentuated in adults compared to Pan (all specimens to 1 cm scale).

Fig. 3.

Hypothesized biomechanical hand posture models describing morphological variation found between Gorilla and Pan and Gorilla carpus in doral view. Lateral view of wrist and hand postures in Pan and Gorilla adapted from ref. . In Pan, the wrist (and carpometacarpal) joints are held in an extended posture (dotted line) such that extension-limiting morphological features are required for stability. In contrast, we hypothesize that Gorilla uses a columnar, neutral wrist and hand posture with axial loading (dotted line) such that stabilizing features are not necessary and generally absent (see text for discussion). Radiocarpal and midcarpal joints are labeled in lateral and dorsal views of Gorilla carpus. “S,” scaphoid; “C,” capitate; “H,” hamate.